1. Field of the Invention
This invention relates to the micromachining of silicon structures in general and to the manufacture of a buried boss or stiffener structure associated with a pressure sensor diaphragm in particular.
2. Description of the Prior Art
Many micromechanical silicon devices are now well known, including sensors for sensing force, pressure, acceleration, chemical concentration, etc. Such devices are termed "micromechanical" because of their small dimensions on the order of a few millimeters square. Such sizes are achieved by utilizing a photolithographic technique similar to that employed in the fabrication of integrated circuits. Silicon wafers well known in the integrated circuit technology can also be used for micromachined structural elements and have the advantage that additional control or sensing electronic circuitry can be formed in conjunction with the structure providing the sensing, in order to process the resultant electrical signal.
Micromachined silicon is well known in a number of different applications. Many operational devices depend upon the flexing of a thin area of silicon which connects relatively thicker areas of silicon, the so-called "boss" areas. For example, in a pressure transducer, there may be a large, relatively stiff, silicon diaphragm boss bounded by a thin, relatively flexible, area along its periphery which permits the diaphragm to move and the thin area to flex depending upon pressure differentials on the sides of the diaphragm. A detailed discussion of the use of relatively thin flexure areas and relatively thick boss areas for pressure or other type silicon sensors is contained in "Low Pressure Sensors Employing Bossed Diaphragms and Precision Etch-Stopping" by Mallon, Barth, Pourahmadi, Vermeulen, Petersen and Bryzek presented on Jun. 25, 1989 at the International Conference on Solid State Sensors and Actuators in Montreux, Switzerland, herein incorporated by reference.
A schematic of a conventional pressure transducer diaphragm is shown in FIG. 1, wherein an upper silicon substrate 10 is fusion bonded to a lower substrate 12 in a vacuum. A cavity 14 forms a chamber which can provide for the maintenance of a vacuum on one side of diaphragm 16 with external pressure being present on the other side of the diaphragm. Alternatively, a differential pressure sensor could be provided by conducting one pressure to the cavity and conducting a second pressure to the external area of the diaphragm. An accelerometer could also be provided by utilizing a fixed weight mass on the diaphragm with sufficient apertures in the lower substrate 12 so as to prevent any pressure differential across the diaphragm.
In the pressure transducer illustrated in FIG. 1, the diaphragm is maintained in an essentially planar form by the use of an increased thickness x extending across the area of the diaphragm. However, in order to concentrate the stress created by deflection of the diaphragm, narrow grooves form flexures 18. As the diaphragm deflects from its initial position to a deflected position (shown in dotted line form) a uniaxial stress is created and concentrated in a direction parallel to the width of the groove.
As is discussed in U.S. Pat. No. 4,904,978 to Barth et al, a piezoresistive area can be provided in the vicinity of the flexure which will sense the level of stress at the flexure, thereby providing an electrical indication of pressure on the diaphragm. The utilization of the thickened portion of the diaphragm (relative to the thin flexure portion) or "boss" structure serves to insure not only uniaxial stress in the flexure but also a relatively linear response to pressure changes on the diaphragm and avoids deformation of the diaphragm itself by limiting its displacement.
In the past, such thin flexures have been created as a result of a boss or reinforcement structure being buried or created in the areas where the thickened structure is desirable, i.e. diaphragm 16 and the boss 20 between flexures 18. Such a method is illustrated in FIGS. 2a through 2c where a substrate of p-type doped silicon 30 is provided. A deep diffusion of n-type impurities is provided which extends to a depth of x which may be 10 to 20 .mu.m (17 .mu.m in a preferred embodiment). This depth is equal to the desired thickness of the resultant boss or diaphragm structure.
A second, shallower and more general n-type diffusion is made as shown in FIG. 2b to a depth of y which will be the thickness in the flexure region which in a preferred embodiment may be 5 .mu.m. The final step is utilization of an etchant process (such as electrochemical-potassium hydroxide (KOH) etching) which will selectively etch the p-type material which provides the resultant structure shown in FIG. 2c where the bosses have a thickness of x and the flexure areas have a thickness of y.
A problem existing in the creation of the thin flexure areas by a generalized or shallow n-type diffusion is that such a diffusion naturally leads to a non-uniform dopant concentration in the flexure. Obviously, there will be a very high concentration of n-type dopants at the surface but this concentration decreases as the depth below the surface increases. As the depth y is approached, the n-type dopant concentration in the flexures 18 will also vary at different lateral positions having the same depth below the surface of the substrate. The consequence is that when the p-type material is etched away, the boundary between the p-type and n-type dopants in the silicon substrate is not at a uniform depth and differing thicknesses of the flexure material will be present at different lateral locations.
Where the resultant thickness of the boss is on the order of 17 to 20 microns, a variation of perhaps 0.5 microns has very little effect.
However, where the thickness of a flexure area is S microns or less, a variation in thickness of 0.5 microns represents a change of at least 10% or higher. Consequently variations in thickness of the flexure areas will result in non-linearities in the performance of the sensor product.